1. Field of the Invention
The present invention relates to a photoelectric conversion device and more particularly to an amplification-type photoelectric conversion device employing a field-effect transistor.
2. Related Background Art
The known semiconductor photoelectric conversion devices are broadly divided into two types. They are as follows:
a) The type in which the charges generated by incident light are delivered as such to the output; Photodiodes, etc., may be cited as typical examples of this type. PA1 b) The type in which the charges generated by incident light produce secondary charges in accordance with the charge quantity thereof and the secondary charges are delivered to the output; In this type of photoelectric conversion devices, the charges generated secondarily are generally amplified as compared with the charges generated by the incident light. Here, this type of photoelectric conversion device is referred to as an "amplification-type photoelectric conversion device". The amplification-type photoelectric conversion devices include for example phototransistors, static induction-type photoelectric conversion devices and photoelectric conversion devices comprising junction field-effect transistors (JFET).
The photoelectric conversion device may be used singly or alternatively a plurality of the photoelectric conversion devices may be utilized as an integrated device so as to form a plurality of picture elements arranged one-dimensionally or two-dimensionally. The latter is generally known as a line sensor or image sensor and in this case the sensor incorporates not only a plurality of photoelectric conversion devices forming a plurality of picture elements but also a charge transfer device, e.g., a CCD and wiring for delivering the signal charges produced by the photoelectric conversion to the outside, a drive circuit for applying a driving signal to each of the respective electrodes of the sensor to drive it, etc.
With the conventional amplification-type photoelectric conversion device utilizing a JFET, the drain voltage is maintained constant throughout all of the charge accumulation period, the signal read-out period and the undesired charge discharge period. On the other hand, in the case of the n-channel JFET, for example, the voltage applied to the gate electrode is maintained at a low level during the charge accumulation period so that a depletion layer is formed in the channel between the source and the drain and the current flow is cut off between the source and the drain. As a result, the holes generated by the photoelectric conversion are accumulated as signal charges and, on the other hand, the electrons are discharged to the substrate of the device. The voltage applied to the gate electrode is raised to a certain intermediary level during the signal reading period so that the depletion layer of the channel is partially extinguished. As a result, the electrons from the source pass through the channel and flow into the drain through the depletion layer in the vicinity of the drain. At this time, the resistance value of the channel assumes a value corresponding to the amount of accumulated holes and therefore the electrons passed through the channel are delivered as the amplified secondary signal charges from the drain to the output terminal. The voltage applied to the gate electrode after the signal reading period is again restored to the low level and therefore the current flow is again cut off between the source and the drain. Thereafter, the voltage applied to the gate electrode is raised to a high level so that the undesired accumulated charges (holes) are recombined with the electrons in the substrate and are lost.
Thus, the amplification-type photoelectric conversion device is capable of outputting the secondary signal charges amplified in accordance with the amount of the charges generated by the incident light and hence is capable of producing a large output signal as compared with the photoelectric conversion device, e.g., the photodiode belonging to the previously mentioned type a). However, the charge amplification factor attained by the conventional amplification-type photoelectric conversion device is still insufficient and generally an output amplifier is incorporated in the output stage of the device so as to obtain the output signal level required for the sensor. If such output amplifier is present, not only the signal component but also the noise component contained therein are amplified and therefore it is impossible to obtain any advantage with respect to the signal-to-noise ratio.